"Fairy tales are ground zero," he said. "They're the first stories we hear... [but] will Chewbacca show up in Storybrooke? Probably not, because that's a Lucasfilm property."

Horowitz also confirmed that the show's writers have to maintain certain standards when portraying Disney characters, such as Cinderella.

"You get the sense of where the line is and you try to push as far over that as you can without going too far," he explained. "We want to do cool dark stories, but we don't want to sully the characters.

"We had things like the Red Riding Hood episode where she kills her boyfriend and eats him. We're getting families to watch this, but we were able to get the dark things we wanted to do."

The second season of Once Upon a Time launched last night in the US. In the UK, Channel 5 is expected to air the new season in early 2013.